Lecture 2: Variables & Assignments

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1 Lecture 2: Variables & Assignments (Sections ,2.5) CS 1110 Introduction to Computing Using Python [E. Andersen, A. Bracy, D. Gries, L. Lee, S. Marschner, C. Van Loan, W. White]

2 CS 1110, Lecture 2 Announcements Sections - Start this week! Yay! - Please go only to the Section you are enrolled in - Need to Change your Section or your Lecture? See our Section Swapping Station on Piazza: https://piazza.com/class/jckqwmqflaz6i?cid=10 Enrollment - Lots of turnover in the first week. Don t give up! - Perhaps another class meets your needs? 2

3 Things to do this week Lab 1: Read textbook Chapter , 2.5 Chapter Go to your registered section Complete lab handout You have one week to show your work: to TA by end of lab, or: in consulting hours up to 1 day before your lab, or: in TA (not professor) office hours (but student questions take precedence over this) to TA within first 10 minutes of next week s lab 3

4 Helping you succeed in this class Consultants. ACCEL Lab Green Room Daily office hours (see website) with consultants Very useful when working on assignments ENGRG 1010: AEW Workshops. Additional discussion course open to ALL students Runs parallel to this class optional See website Piazza. Online forum to ask/answer questions Go here first before ing questions Office Hours. Talk to the professors! Olin 128 between lectures 4

5 iclickers Fun! Interactive! Not for a grade; no need to register. 5

6 iclicker Warm Up: Using Python How do you Plan to use Python? A. I want to work mainly in the computer labs B. I want to use my own Windows computer C. I want to use my own Macintosh computer D. I want to use my own Linux computer E. I will use whatever I can get my hands on 6

7 From last time: Types Type: set of values & operations on them Type float: Values: real numbers Ops: +, -, *, /, ** Type int: Values: integers Ops: +, -, *, //, %, ** Type bool: Values: integers Ops: not, and, or Type str: Values: string literals Double quotes: abc Single quotes: abc Ops: + (concatenation) 7

8 Converting from one type to another Command: <type>(<value>) >>> float(2) 2.0 converts value 2 to type float converts value 2.6 to type int >>> int(2.6) 2 This kind of conversion is also called casting Different from type(<value>) type(<value>) tells you the type <type>(<value>) converts the type 8

9 Implicit (Automatic) Conversions Python sometimes converts types automatically Example: 1/2.0 - evaluates to a float: internally: Step 1: Python casts 1 (an int) to 1.0 (a float) Step 2: Python evaluates 1.0/2.0 Behavior depends on whether the conversion is narrowing or widening 9

10 Variable width Types differ in how much information held Convert without losing information? floatàint (e.g., 4.7 to 4) intàfloat (e.g., 4 to 4.0) info lost seems ok Wide = more information capacity From narrow to wide: bool int float 10

11 Widening Conversion From a narrower type to a wider type Python does automatically if needed: Example: 1/2.0 evaluates to a float: 0.5 Example: True + 1 evaluates to an int: 2 True converts to 1 False converts to 0 Note: does not work for str Example: 2 + ab produces an error 11

12 Narrowing Conversion From a wider type to a narrower type Example: int(2.6) causes information to be lost Python never does this automatically Note: you can just always cast Instead of 1/2.0, can write float(1)/2.0 12

13 Types matter! You Decide: What is the right type for my data? When is the right time for conversion (if any) Zip Code as an int? Grades as an int? Lab Grades as a bool? What are your goals: Accuracy? Clarity? Fairness? 13

14 Operator Precedence What is the difference between: 2*(1+3) 2*1 + 3 add, then multiply multiply, then add Operations performed in a set order Parentheses make the order explicit What if there are no parentheses? à Operator Precedence: fixed order to processes operators when no parentheses 14

15 Precedence of Python Operators Exponentiation: ** Unary operators: + Binary arithmetic: * / % Binary arithmetic: + Comparisons: < > <= >= Equality relations: ==!= Precedence goes downwards Parentheses highest Logical ops lowest Same line = same precedence Read ties left to right (except for **) Example: 1/2*3 is (1/2)*3 Logical not Logical and Logical or Section 2.5 in your text See website for more info Major portion of Lab 1 15

16 Operators and Type Conversions Evaluate this expression: Operator Precedence Exponentiation: ** Unary operators: + Binary arithmetic: * / % Binary arithmetic: + Comparisons: < > <= >= Equality relations: ==!= Logical not Logical and Logical or 1/31/17 Variables & Assignments False / 3 A. 3 B. 3.0 C D. 2 E

17 Operators and Type Conversions Evaluate this expression: Operator Precedence Exponentiation: ** Unary operators: + Binary arithmetic: * / % Binary arithmetic: + Comparisons: < > <= >= Equality relations: ==!= Logical not Logical and Logical or 1/31/17 Variables & Assignments False / 3 False

18 New Tool: Variable Assignment An assignment statement: takes an expression evaluates it, and stores the value in a variable Example: variable >>> x = 5 equals sign (just one!) (read right to left) expression evaluates to 5 18

19 Executing Assignment Statements >>> x = 5 >>> Press ENTER and Hmm, looks like nothing happened But something did happen! Python assigned the value 5 to the variable x Internally (and invisible to you): memory location x 5 stored value 19

20 Retrieving Variables >>> x = 5 >>> x 5 >>> Press ENTER and Python tells me the stored value 20

21 In More Detail: Variables (Section 2.1) A variable is a named memory location (box) contains a value (in the box) Examples: Variable names must start with a letter (or _). x area Variable x, with value 5 (of type int) Variable area, w/ value 20.1 (of type float) The type belongs to the value, not to the variable. 1e2 is a float, but e2 is a variable name 21

22 In More Detail: Statements >>> x = 5 >>> Press ENTER and Hm, looks like nothing happened This is a statement, not an expression Tells the computer to DO something (not give a value) Typing it into >>> gets no response (but it is working) 22

23 Expressions vs. Statements Expression Represents something Python evaluates it End result is a value Examples: 2.3 (3+5)/4 x == 5 Value Complex Expression Statement Does something Python executes it Need not result in a value Examples: x = x = 5 Look so similar but they are not! 23

24 You can assign more than literals >>> x = 5 >>> x = 3.0 ** >>> x = 2 + x x gets 5 x gets the value of this expression x gets 2 plus the current value of x 24

25 Keeping Track of Variables Draw boxes on paper: >>> x = 5 New variable declared? >>> y = 3 Write a new box. x 5 y 3 7 Variable updated? >>> x = 7 Cross out old value. Insert new value. 25

26 Execute the Statement: x = x + 2 Draw variable x on piece of paper: x 5 1. Evaluate the expression x + 2 For x, use the value in variable x Write the expression somewhere on your paper 2. Store the value of the expression in x Cross off the old value in the box Write the new value in the box for x Did you do the same thing as your neighbor? If not, discuss. 26

27 Which one is closest to your answer? A. x 5 7 B. x 5 x 7 C. x 5 x 7 D. \_( )_/ x = x

28 And The Correct Answer Is A. x 5 7 B. x 5 x 7 C. x 5 x 7 D. \_( )_/ x = x

29 Execute the Statement: x = 3.0*x+1.0 Begin with this: x Evaluate the expression 3.0*x Store its value in x Did you do the same thing as your neighbor? If not, discuss. 29

30 Which one is closest to your answer? A. B. x 5 7 x x 22.0 C. x 5 7 x 22.0 D. \_( )_/ x = 3.0*x

31 And The Correct Answer Is A. B. x 5 7 x x 22.0 C. x 5 7 x 22.0 D. \_( )_/ x = 3.0*x

32 Executing an Assignment Statement The command: x = 3.0*x+1.0 Executing the command : 1. Evaluate right hand side 3.0*x Store the value in the variable x s box Requires both evaluate AND store steps Critical mental model for learning Python 32

33 Exercise 1: Understanding Assignment Begin with: Declare a new variable: >>> rate = 4 x rate 4 Execute this assignment: >>> rate = x / rate Did you do the same thing as your neighbor? If not, discuss. 33

34 Which one is closest to your answer? A. x rate B. x rate 4 rate 5.5 C. D. x x rate rate 4 5 E. \_( )_/ rate = x / rate 34

35 And The Correct Answer Is A. x rate B. x rate 4 rate 5.5 C. x D. x rate rate 4 5 rate = x / rate 35

36 Dynamic Typing Python is a dynamically typed language Variables can hold values of any type Variables can hold different types at different times The following is acceptable in Python: >>> x = 1 >>> x = x / 2.0 ç x contains an int value ç x now contains a float value Alternative: a statically typed language Examples: Java, C Each variable restricted to values of just one type 36

37 More Detail: Testing Types Command: type(<value>) Can test a variable: >>> x = 5 >>> type(x) <class 'int > Can test a type with a Boolean expression: >>> type(2) == int True 37

38 Exercise 2: Understanding Assignment Begin with: x rate Execute this assignment: >>> rat = x + rate Did you do the same thing as your neighbor? If not, discuss. 38

39 Which one is closest to your answer? A. x rate B. x rate rat 27.5 C. x D. x rate rate rat 27.5 E. \_( )_/ rat = x + rate 39

40 And The Correct Answer Is A. x rate B. x rate rat 27.5 C. x D. x rate rate rat 27.5 Spelling Matters! rat = x + rate 40

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